Bontrager
Interchange Rear Trunk Deluxe Bag

Details

The Interchange Rear Trunk Deluxe makes it easy to bring everything you need. It attaches to and removes from Bontrager's Interchange Racks (sold separately) in seconds and installs to most other racks with Velcro straps. Plus, it boasts 700 to 1000 cubic inches of expanding storage space to carry ride essentials or the makings of a great picnic. And, the Interchange's aerodynamic shape fits nicely and looks great.

This bag has been my best investment into bike commuting. I wanted a rear trunk that mounted on top of my Bontrager rear rack so I wouldn't have to mess with the balance issues of a pannier bag. I keep a few essentials(shoes, belt, etc.) at work, and this bag holds my daily clothing plus a small lunch bag without issue. The Interchange attachment works awesome with a solid fit and an easy push-button release, so I can take the whole thing with me to my locker at work without having to detach and reattach Velcro straps of other trunk/rack designs. Also not mentioned in the product description is that it has an attached water resistant fitted rain cover that is nicely hidden away in a small zippered pouch. Both the bag and the rain cover have rear straps to mount rear tail lights.
This Deluxe bag works great for me, I considered the base Rear Truck Interchange bag they offer, but I love the expandability of the Deluxe to flex with my commuting storage needs. I did not bite on the Deluxe Plus model which has temporary Panniers built into the side pockets, but in reading the reviews there, it might be worth checking it out if your bike commute usually includes other errands.